The present invention relates to an optical transmitting apparatus and an optical receiving apparatus, and an optical communicating method suitable for use in an optical communicating system for an optical communication network utilizing broad band characteristics of fibers.
The optical communication network can transmit a large volume of information as compared with a network using general copper cables. In recent years, the optical communication network draws attention as a network which provides a multi-media service such as CATV (Cable Television), VOD (Video on Demand) or the like.
FIG. 26 is a block diagram showing an example of an optical communicating system for providing a multi-media service. In an optical communicating system 100 shown in FIG. 26, a single station 101 is connected to an optical coupler 102 on a transmission route through one optical fiber 103, and a plurality (n, here) of subscriber terminals 104-1 through 104-n are connected to the optical coupler 102 through respective exclusive optical fibers 103-1 through 103-n.
Between the station 101 and each of the subscriber terminals 104-1 through 104-n, optical signals are transmitted and received in wavelength division multiplexing. Namely, the station 101 can multiplex signals of wavelengths .lambda. 1 through .lambda. n, and transmit and receive the signals through the optical fiber 103.
The optical coupler 102 demultiplexes the wavelength division multiplexed optical signal fed through the optical fiber 103 into optical signals of wavelengths .lambda. 1 through .lambda. n, and transmits the demultiplexed optical signals to the subscriber terminals 104-1 through 104-n through the optical fibers 103-1 through 103-n, respectively.
The optical coupler 102 couples optical signals (having wavelengths .lambda. 1 through .lambda. n, respectively) each of a single wavelength incident from the subscriber terminals 104-1 through 104-n through the respective optical fibers 103-1 through 103-n, and outputs the optical signals as a wavelength division multiplexed signal to the optical fiber 103 on the side of the station 101.
The above optical communicating system 100 as shown in FIG. 26 is configured between the station 101 and the subscriber terminals 104-1 through 104-n, whereby a routing (data transmission) in a wavelength for each subscriber and a band for each subscriber can be increased.
However, the optical communicating system 100 using the above wavelength division multiplexing technique as shown in FIG. 26 has a disadvantage such that the number of subscribers is limited to the number of wavelengths which can be multiplexed. If it is desired to increase the number of subscribers, it is considered an optical communicating system 100A in which the above wavelength division multiplexing system employed in FIG. 26 is combined with a time division multiplexing system.
Namely, in the optical communicating system 100A employing the time division multiplexing system as shown in FIG. 27, transmit-receive signals for three subscriber terminals 104-u, 104-v and 104-w (u, v, w; different integers from 1 to n) are assigned to optical signals having the same wavelength .lambda. i, and each of the subscriber terminals 104-u, 104-v and 104-w extracts a signal addressed to itself from the optical signals of the same wavelength .lambda. x (x; an integer from 1 to n) from the station 101 using a unique clock signal.
However, the above optical communication network shown in FIG. 27 has a low extensibility of the network to increase the number of subscribers of the optical communicating system. In addition, there is a possibility that the subscriber terminals 104-u, 104-v and 104w may receive a signal addressed to another terminal depending on setting of a clock signal or the like. Therefore, there is required an improvement in security (so-called privacy protection).
In the light of the above problems, an object of the present invention is to provide an optical transmitting apparatus and an optical receiving apparatus, and an optical communicating method, which can accommodate subscribers larger in number than wavelengths multiplexed, while realizing privacy protection.